A method of sending a liquid by a peristaltic movement which occurs at an interface with a liquid inside a channel by generating a progressive wave in a flexible member which constitutes at least a part of a wall surface of the channel has been proposed (Patent Document 1, Non-patent Document 1). In a pump unit in which, the abovementioned method has been used, a complicated mechanism such as a chuck valve is unnecessary, and therefore it is suitable for small-sizing. When a small-size pump unit is realized, by disposing the small-size pump unit at a front-end portion of an endoscope, it is possible to realize a simple and highly reliable cooling mechanism which circulates a cooling medium in a closed-loop channel which is extended rearward from the front-end portion of the endoscope.